1. Field of the Invention
The present invention relates to a method for curing dental resins for the purpose of obtaining dental normal-temperature polymerizable resins, which are remarkably improved in terms of transparency, color stability and physical properties.
2. Statement of the Invention
About fifty years ago, that is in 1936, that one started to use polymethyl methacrylate mainly in dentistry and stomatoplasaty. Representative of synthetic resins resulting from the polymerization of polymerizable compounds having ethylenically unsaturated double bonds is polymetyl methacrylate. The application of methyl methacrylate and polymethyl methacrylate to dentistry has had its origin in the invention of the powder/liquid blending technique established in 1936. This technique has the advantage that the reactive catalyst can be supplied in two portions, viz., a powder portion and a liquid portion.
In general, the polymerization of methyl methacrylate is broken down into thermal polymerization and normal-temperature polymerization. The thermal polymerization involves the activation of peroxides by heat, and gives polymers through a succession of reactions of initiation, growth and interruption. In the normal-temperature polymerization, on the other hand, the action of promotors becomes important rather than the action of heat. The promotors, which were discovered in 1941 for the first time, have been used up to date and little improvement has been made since. Among others, the combination of dimethyl p-toluidine that is an aromatic tertiary amine with benzoyl peroxide that is an organic peroxide is found to be of the most general-purpose properties. However, this combination has the following grave disadvantages.
The first disadvantage is that the reaction product remaining in the cured synthetic resin turns yellow upon exposure to light.
The second is thata discoloration or coloration of the cured product occurs due to contacting sputum or exposure to the mouth temperature or in the intramouth environment including food refuses.
The third is that unreacted part of the promotor and derivatives not formed in the chainlike molecules act together as a plasticizer which causes a lowering of the physical properties of the polymerized synthetic resin.
The fourth is that the heat of polymerization is high.
The fifth is that the cured product is deficient in transparency.
The first disadvantage may be eliminated to some extent by the addition of an ultraviolet absorber. Not until now, however, is the second disadvantage practically overcome. What is more, the second disadvantage involves the coloration of the cured product which is originally caused by the inherent properties of the promotor and the so-called discoloration that is a gradual change in color. In an effort to overcome the third disadvantage, the plasticizer effect of the residual promotor is improved by making use of various types of polymers, in particular, methyl ester polymers in an increased amount. However, such an effort has still its own limit.
For normal-temperature polymerizable resins for dental purposes, certain combinations of benzoyl peroxide with aromatic tertiary amines have generally been used as catalysts. However, the obtained dental resins are poor in color stability and, in particular, suffer from noticeable yellowing. For instance, when the oral cavity restoration material obtained by this method is inserted into the mouth of a paatient, it has to be adjusted to a color as natural as her or his intramouth environment with much effort. Nonetheless, yellowing is initiated within a short period of time with the result that the restoration material assumes a quite unnatural color and loses aesthetic properties. Further, the so-called plasticizer effect of the residual promotor causes a lowering of the physical properties of the restoration material due to large amounts of the unreacted benzoyl peroxide or aromatic tertiary amines remaining therein. The fourth defect is that when the polymerizable compounds having at least one ethylenically unsaturated double bond are polymerized into a rebasing material applied directly in the mouth in the presence of the conventional catalyst, viz., a benzoyl peroxide/aromatic tertiary amine combination, the polymerized or cured product causes irritative pain to the mouth of a patient because of a large amount of heat generated during polymerization. The fifth defect, insufficient transparency of the cured product, remains substantially unsolved.